How do I make a textures values translate into shader values
The Alpha Channel works, because it is read linear. No Gamma correction etc. Color spaces HSL, HSV etc. can be very complex, so my suggestion is as @Jachym Michal points out, non-color, or linear color. However keep in mind, the roughness values of the materials are squared now, so they are less sensitive than in 2.79, in case you want to work with precize values. Lastly, you can use a color ramp to map the 2 values (0.75 and 1) to any color you want. That might save you going back and forth between GIMP and Blender.